Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard that permits the addition of high-speed data transfer over an existing cable TV (CATV) system. It is employed by many cable television operators to provide Internet access over their existing hybrid fiber-coaxial (HFC) infrastructure.
DOCSIS was developed by Cable Television Laboratories, Inc. (a/k/a “CableLabs”) and a number of contributing companies in the late 1990's. The first specification, version 1.0, was issued in March 1997. The latest specification, DOCSIS 3.0, was released in August 2006 and significantly increased transmission speeds, both upstream and downstream, while introducing support for Internet Protocol version 6 (IPv6).
The next specification, DOCSIS 3.1, is in development and is expected to be field tested sometime in 2014. The DOCSIS 3.1 standard uses orthogonal frequency-division multiplexing (OFDM) to achieve at least 10 Gbit/s downstream and 1 Gbit/s upstream transmission rates.
With each successive specification the upstream bandwidth has increased. This requires ever-greater power to be produced by the cable modems to drive the DOCSIS upstream amplifiers which must amplify multiple digitally modulated channels simultaneously while keeping spurious distortion products very low, typically more than 50 dB below the carrier level.
To achieve the low distortion levels, power-hungry Class A amplification is typically used for DOCSIS upstream amplifiers. This is because combining multiple digitally modulated carriers creates signals with high peak-to-average ratios. Since the peaks must be faithfully reproduced to minimize bit error rate (“BER”), the amplifier must be biased to accommodate the peaks, resulting in high power dissipation. This is not desirable from a number of perspectives including power consumption, heating of the cable box, and degrading the circuitry of the cable box over time.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.